The last 15 years have seen dramatic advances in the technology for acquisition of brain images. Magnetic resonance imaging (MRI), positron emission tomography (PET), and x-ray computed tomography (CI) produce scores of cross-sectional images which play a crucial role in diagnosis and therapy. Three-Dimensional models of the patient's brain, created from these data, can be used to Perform computer simulations of neurosurgical procedures or radiation treatments. However, these methods of therapy planning have suffered a missing link namely, the absence of a convenient quantitative method of transferring these highly precise treatment plans from images to the patient's body.The PIs have recently developed an interactive localizer which bridges this gap by registering images with the patient's anatomy in a non-invasive, retrospective manner. The further development of this type of frameless stereotaxy will make it possible to perform fully quantitative image-guided neurosurgery and radiation therapy. Such methods may be less time consuming, less expensive, more convenient, and entail less post-procedure morbidity than conventional qualitative techniques. Specific Aims 1. Construction of a mobile localizer which is suitable for use in inpatient hospital rooms, clinics, operating rooms, or radiation treatment suites. 2. Development of applications software for neurosurgical planning and radiotherapy planning. 3. Testing of the neurosurgical localizer on phantoms, volunteers, and brain tumor patients. 4. Testing of the radiotherapy localizer on phantoms, volunteers, and brain tumor patients.